Acid aqueous solutions of chlorites



Patented Sept. 26, 1944 UNETE STA ACID AQUEOUS SOLUTIONS OF CHLORITES James Douglas MacMahon, Niagara Falls, N. Y., assignor to The Mathieson Alkali Works, Inc., New York, N. Y., a corporation of Virginia No Drawing. Application August 13, 1943,

Serial No. 498,586

8 Claims.

used with advantage in various commercial operaqueous Solutions- I have discovered that the ations, for example in pulping and bleaching in formation of chlorine dioxide in such solutions the paper industry, in the bleaching of other may be materially repressed or even substantially various materials including textiles and the like i nat d, w t ut deleteriously affecting and in various oxidation processes. In many utility of t e Solution, y the Presence of hydroinstances, it has been found particularly advan- 10 g peroxidetageous to use the chlorites in acid aqueous solu- I have discovered that, by the addition of hytions. However, such use has heretofore fredrogeh Peroxide as Such or of Compounds 1 quently been accompanied by certain disadvanable of yielding hydrogen peroxide in the e tages vironment of the solution, for example sodium It is known that in acid aqueous solutions the pe bo ate, sodium percarbona e and the like, the chlorites decompose to greater or less t t evolution of chlorine dioxide under moderately t for chlorine dioxide, There has been acid conditions may be either partially or comsiderable speculation as to the relative effectivep y rep and t under Such Conditions, ness of the chlorine dioxide and the chlorite ion, e yd o peroxide either does not reeet With respectively, in such operations. Howev r, djfiithe chlorite ion at all or, if so, reacts only to such culties arising out of the volution of hlori a limited degree as to result in no interference dioxide in such operations are well recognized. With the latter as a bleaching 0r oxidizing a Frequently commercial operations using acid The invention thus provides practical means aqueous chlorite solutions are conducted in metal whereby the Concentration of chlorine dioxide n tanks or vats. Chlorine dioxide is Very col-rm the solution may be controlled to a considerable sive and even where vessels fabricated of certain extent d e rate t W h 2 is generated types of stainless steel are used, excessive evolumay be so regulated as to Prevent the escape of tion of chlorine dioxide tends to cause corrosion any considerable quantities of C102 from the 50111" or pitting of the metal surfaces, particularly portiontions thereof exposed to the vapor phas adja, I cannot state With. certainty the precise chemm, t th surface of t l t Accgrdingly ical reaction by which the formation of chlorine the evolution of chlorine dioxide is frequently dioxide is repressed- However, the utility Of y objectionable from the standpoint of corrosion. invention is not dependent upon any precise Further, the escape of chlorine dioxide gas from understanding of the Principle involved d I the solution represents a loss in bleaching or oxido not intend to be bound to any theory as to dation value and, if excessive, may also result in its operatiorl- In referring herein t e formaobjectionable atmospheric conditions in the vition, generation O evolution of Chlorine diOXide, cinity of the operation. I do not intend to distinguish between a condition Also, excessive concentrations of chlorine diwhere the actual formation of chlorine dioxide is oxide tend to cause a discoloration f anim l retarded or prevented and a condition where the fiber such as w nd silk, ll tending t chlorine dioxide may be formed as such but is so ward a reddish or yellowish cast. Also, in the rapidly reconverted to the chlorite or converted bleaching of sponges, using acid, chlorite ol to some unobjectionable compounds as to repress tions, excessive concentrations of chlorine dioxide the accumulation of chlorine dioxide in the solumay result in color instability of the sponge. tier!- Further, in laundry wash rooms, where chlorites It has been known that alkaline hyd are used in the souring step of commercial launperoxide reacts with chlorine dioxide to reduce dry operations t bi souring and bl hi the chlorine valence of the latter to that of the which rooms are frequently congested and inchlorite 1011. It has 8180 been known that, under adequately ventilated, the escape of substantial extreme'eoid conditions, beleW D orine quantities of chlorine dioxide into the atmosdioxide a y m n peroxide do not react with phere is particularly objectionable both from the one another at normal temperatures. But, so standpoint of the personnel and from the standfor as I am aware, it has not been known herepoint of possible corrosion of metal equipment, tofore that the evolution of C102 could be re- In accordance with my present invention, the evolution or formation of chlorine dioxide may be repressed so as to avoid the difficulties heretofore experienced in the use of chlorites in acid pressed in an acid aqueous chlorite solution without substantial interference with the bleaching or oxidizing action of the solution by the presence of hydrogen peroxide.

The extent to which the evolution of chlorine dioxide is repressed in accordance with my invention varies with particular operating conditions including temperature and concentration of hydrogen ions, chlorite and hydrogen peroxide in the solution. At a constant temperature, for example 80 C., the extent of repression decreases with an increase in hydrogen ion concentration, i. e., a decrease in the pH of the solution. At a constant pH, as for example pH 4, the extent of the repression decreases with an increase in temperature. However, over an observed broad range of variations of temperature and concentrations, I have found that the presence of hydrogen peroxide does not materially interfere with the bleaching or oxidizing action of the chlorite ions.

A possible explanation of the results obtained is that the oxidation potential of hydrogen peroxide is sufficiently lower than that of chlorine dioxide to induce an appreciable reaction between them whereas the difference in oxidation potential of hydrogen peroxide and of the chlorite ion is not sufiicient to cause them to react to any significant extent. According to this theory, it would appear that the concentration of chlorine dioxide existing in the solution at any given time depends upon the balance that is maintained between the rate at which the chlorine dioxide is reduced by the hydrogen peroxide and the rate at which the chlorite ion is converted to chlorine .dioxide. While not binding myself to this theory, I have found it useful in interpreting my experimental results.

As previously noted, the extent of the repression of C102 evolution by the presence of hydrogen peroxide varies appreciably with the particular operating conditions employed. The ellect of these variables appears from the following specific illustrations of the application of my invention under the indicated operating conditions. For comparison, the amounts 01 C102 evolved under similar conditions where the H202 is omitted are also shown.

In each of the following examples, I to VIII inclusive, an equal volume (500 milliliters) of acid aqueous sodium chlorite solution was used and the repressive effect of the hydrogen peroxide under the various conditions, measured in terms of the number of milligrams of chlorine dioxide generated in a period of thirty minutes. The amount of C102 generated was determined by aerating the C102 from the solution into an absorption tower containing a potassium iodide solution and then titrating the solution from the tower with a standard thiosulfate solution.

Example I The efiect of variations in hydrogen ion concentration on the repressive action of the hydrogen peroxide is illustrated by a series of operations in which acid chlorite solutions of equal sodium chlorite concentration, in some of which no hydrogen peroxide was present and to others equal amounts of hydrogen peroxide were added, were tested at pH values of from 2 to '7. In each of these tests, a temperature of 80 C. was maintained and the initial concentration of sodium chlorite in each was equivalent to 2 grams per liter of available Cl2. The initial concentration of hydrogen peroxide in the solutions to which it was added was in each instance equivalent to 2 grams per liter of IOU-volume H202. The number of milligrams of C102 generated in 30 minutes in the respective operations was as indicated in the following tabulation:

From the results of these tests it appears that in the acid solutions varying in pH from 2 to '7, the generation of C102 was materially repressed, that at pH values of 5 and higher, only very minor amounts of C102 were evolved and that a pH of 7 no measurable amount of C102 was evolved.

Even in more strongly acid solutions, I have found the presence of hydrogen peroxide to repress materially the generation of 0102. The repressive action of hydrogen peroxide in solutions having a pH of 1, is illustrated by the following example:

Example II In each of two acid aqueous solutions of sodium chlorite the initial concentration of chlorite was equivalent to 2 grams per liter of available chlorine. To one, the equivalent of 2 grams per liter of -volume hydrogen peroxide was added. In each the acidity was adjusted to pH 1 and a temperature of 40 C. was maintained for 30 minutes. During this period, 205 milligrams of chlorine dioxide was generated in the solution to which no hydrogen peroxide was added while only 117 milligrams of chlorine dioxide was generated in this same period in the solution in which the hydrogen peroxide was present. The results of these tests show that, at pH values even as low as 1', the presenceof hydrogen peroxide has a substantial repressive effect on the generation of chlorine dioxide even at relatively low temperatures.

Example III The effect of the variation of temperature on the repressive action of hydrogen peroxide on C102 evolution is illustrated by a series of tests on acid aqueous sodium chlorite solutions each having a pH of 4 and an initial concentration of sodium chlorite equivalent to 2 grams per liter of available chlorine. To half of these solutions, hydrogen peroxide was added in amounts equivalent to 2 grams per liter of 100-volume H202. In the others, no hydrogen peroxide was present. The number of milligrams of chlorine dioxide generated in a 30-minute period, under difierent temperature conditions, was as indicated in the following tabulation:

Milligrams G102 generated in 30 min.

Temp. of solution Chlorite- (lhlorite peroxide Ewample IV Milligrams C101 generated in 30 min. Initial concentration sodium chlorite (g. p. 1. available chlorine) Cm t on e- Ohlorlte peroxide Example V The efiect of the ratio of chlorite to H202 present in the solution upon the repressive action of the H202 is illustrated by a series of tests in each of which the acidity of the solution was pH 4, the initial concentration of sodium chlorite was equivalent to 2 grams per liter available chlorine and a temperature of 80 C. maintained. In the respective tests, the number of milligrams of chlorine dioxide generated in a period of 30 minutes in solutions varying in initial H202 content from none up to 8 parts of H202 for each 2 parts of chlorite is indicated in the following tabulation.

Milligrams G. p. 1. available Clz as NaClO: O: gen- G. p. l. 100-vol. H202 erated in 30 min.

Example VI in the following tabulation:

Milligrams G. p. 1. available 01: as NaClOa C102 gen- G. p. l. 100-vol. H202 erated in 30 min.

As previously noted, the repression of C102 evolution may be effected in accordance with my invention either by the addition of hydrogen peroxide as such or by the addition to the solution of a compound, for example sodium perborate, sodium percarbonate or the like, capable of yielding H202 in the environment of the solution. The effectiveness of the use of sodium perborate and sodium percarbonate, respectively, is illustrated by the following examples:

Example VII In a solution containing 4 grams per liter of available chlorine as sodium chlorite, there was generated 103.4 milligrams of chlorine dioxide in 30 minutes at pH 4 and a temperature of C. In a similar solution to which 2 grams per liter of volume hydrogen peroxide was added, there was generated, under the same conditions, 26.9 milligrams of chlorine dioxide. Where 2.48 grams per liter of sodium perborate (NaBO2.H202.3H2O) equivalent to 2 grams per liter of 100 volume hydrogen peroxide, was substituted for the hydrogen peroxide, only 19.5 milligrams of chlorine dioxide was generated during the 30 minute period.

Example VIII To a solution containing 4 grams per liter of available chlorine as sodium chlorite, there was added 1.735 grams per liter of sodium percarbonate Na2CO3.3H2O2), equivalent to 2 grams per liter of 100 volume hydrogen peroxide. Under the same conditions as in the preceding example, only 16.6 milligrams of chlorine dioxide was generated in the 30 minute period.

Furthermore, the hereindescribed advantages of my invention may be attained without loss in efficiency of operation. The amount of bleaching or oxidation which may be accomplished by the use of a given amount of chlorite in accordance with my invention is usually equal to that accomplished in accordance with conventional practices. This is illustrated by the following examples:

Example IX 250 cc, of a solution containing 2 grams per liter of available chlorine as sodium chlorite, and no repressing agent, bleached 25 grams of gray cotton muslin to a brightness of '78 in 1 hour, where the temperature was F. and the hydrogen ion concentration was pH 3, the ratio of solution to cloth being 10:1. Under the same conditions the same cloth was bleached to a brightness of 78 with a solution containing 2 grams per liter of available chlorine as sodium chlorite and 3.52 grams per liter of 100 volume hydrogen peroxide. In each instance the solution also contained 1 gram per liter of a sodium salt of a fatty alcohol sulphate as a detergent.

Ealamzile X A 20 gram sample of an air-dried pulp at 5% consistency and having a brightness of 65 and a pH of 4 Was treated with 2% available chlorine as sodium chlorite, based on the air-dried pulp, for 2 hours at a temperature of 80 C. During this period 3.96 milligrams of chlorine dioxide was generated and the final brightness of the sample was 82. A similar sample of pulp was treated under identical conditions, except that 1 of 100 volume hydrogen peroxide was added and the pulp was bleached to the same brightness with the generation of only 0.61 milligram of chlorine dioxide.

'I'he invention is generally applicable to operations wherein aqueous solutions of the salts of chlorous acids, for example chlorites of the alkali and alkaline earth metals, are used on the acid side and, as illustrated by the foregoing examples, may be used over a wide range of operating conditions to meet the requirements of the particular operation to which it is applied. It has been applied with particular advantage to the use of acid aqueous solutions of sodium chlorite having a pH greater than 1. The invention may be used with advantage, for instance, in the bleaching or solubilizing of starch, in the treatment of cellulosic materials such as cotton, linen, paper pulp, rayon, cellulose acetate and other cellulose esters and ethers including mixed types, for example cellulose acetate-butyrate, the bleaching of straw such as used in hat-making, the treatment of oils, fats and waxes and the treatment of synthetic protein-like fibers, including materials such as nylon, Aralac and the like.

The advantages of my invention as applied to the bleaching and solubilizing of starch are illustrated by the following example.

Example XI A 6.5% suspension of corn starch in water was prepared and the pH of the suspension adjusted to 3.5 by means of a buffer. The suspension was then heated to a temperature of 70 C. for a period of thirty minutes, allowed to cool to 50 C. and the batch divided into four parts. To the separate portions, reagents were added as subsequently indicated and each portion maintained at a temperature of 50 C. for two and one-half hours.

The first portion was used as a control and to this no reagent was added. To the second portion, an amount of sodium chlorite equivalent to 1% available chlorine, on the weight of the starch, was added. To the third portion, an amount of hydrogen peroxide was added equivalent to 0.5% of 100-volume hydrogen peroxide, on the weight of the starch. To the fourth portion, both sodium chlorite and hydrogen peroxide were added in proportions equivalent to those added to portions two and three respectively.

Immediately following the two and one-half hours reaction period and cooling of the respective sample to 30 0., the viscosities of the respective samples were measured. Thereafter the samples were permitted to stand overnight at room temperature and their viscosities redetermined. In each instance the viscosity of those portions of the starch which had been treated with the reagents was substantially less than the viscosity of the control portion. However, the viscosity of the starch which had been treated with the acid aqueous chlorite solution in the presence of hydrogen peroxide, in accordance with the present invention, was reduced to a substantially greater extent than that of either the starch treated with the acid aqueous chlorite solution alone or with the hydrogen peroxide alone.

The samples treated with chlorite alone and with the mixture of chlorite and hydrogen peroxide showed a substantial degree of bleaching. However, the sample treated with hydrogen peroxide alone showed little, if any bleaching efiect.

There was no visible evidence of the presence of chlorine dioxide in the sample that had been treated with the mixture of chlorite and hydrogen peroxide. However, that portion of the starch which had been treated with the acid chlorite solution alone was found at the end of the reaction period to contain chlorine dioxide to such an extent as to impart a yellow color to the starchy mass and in order to observe clearly the whiteness of the starch it was necessary first to eliminate the chlorine dioxide from the mass by" treatment with hydrosulfite. This treatment with hydrosulfite was subsequent to the viscosity determinations.

The hydrogen peroxide may be added as an aqueous solution such as the so-called volume hydrogen peroxide, as 3% solution or any other suitable concentration. The persalts, when used, may be added in the solid form or in solution.

I claim:

1. In the use of acid aqueous solutions containing a chlorite, the improvement which comprises repressing the generation of chlorine dioxide in the solution by the presence therein of H202.

2. In the use of acid aqueous solutions containing a chlorite and having a pH greater than 1, the improvement which comprises repressing the generation of C102 in the solution by the presence therein Of H202.

3. In the use of acid aqueous solutions containing a chlorite selected from the group con sisting of alkali metal and alkaline earth metal chlorites and having a pH greater than 1, the improvement which comprises repressing the generation of 0102 in the solution by the presence therein of H202.

4. In the use of acid aqueous solutions containing sodium chlorite, the improvement which comprises repressing the generation of 0101 in the solution by the addition of hydrogen peroxide thereto.

5. In the use of acid aqueous solutions containing a chlorite selected from the group consisting of alkali metal and alkaline earth metal chlorites and having a pH greater than 1, the improvement which comprises repressing the generation of 0102 in the solution by the addition of hydrogen peroxide thereto.

6. In the use of acid aqueous solutions containing sodium chlorite and having a pH greater than 1, the improvement which comprises repressing the generation of, C102 in the solution by the addition thereto of a compound which in the environment of the solution decomposes to form H202.

7. In the use of acid aqueous solutions containing sodium chlorite and having a pH greater than 1, the improvement which comprises repressing the generation of 0102 in the solution by the addition thereto of sodium perborate.

8. In the use of acid aqueous solutions containing sodium chlorite and having a pH greater than 1, the improvement which comprises repressing the generation of 0102 in the solution by the addition thereto of sodium percarbon'ate.

JAMES DOUGLAS MACMAHON. 

